In May 1813, as the heir presumptive of the kingdoms of Denmark and Norway, Christian was sent as stattholder (the Danish king's highest representative in overseas territories) to Norway to promote the loyalty of the Norwegians to the House of Oldenburg, which had been very badly shaken by the disastrous results of Frederick VI's adhesion to the falling fortunes of Napoleon I of France. Christian did all he could personally to strengthen the bonds between the Norwegians and the royal house of Denmark. Though his endeavours were opposed by the so-called Swedish party, which desired a dynastic union with Sweden, he placed himself at the head of the Norwegian party of independence after the Treaty of Kiel had forced the king to cede Norway to the king of Sweden. He was elected Regent of Norway by an assembly of notables on 16 February 1814.

This election was confirmed by the Norwegian Constituent Assembly convoked at Eidsvoll on 10 April, and on 17 May the constitution was signed and Christian was unanimously elected king of Norway under the name Christian Frederick.

Christian next attempted to interest the great powers in Norway's cause, but without success. On being pressed by the commissioners of the allied powers to bring about a union between Norway and Sweden in accordance with the terms of the treaty of Kiel, and then return to Denmark, he replied that, as a constitutional king, he could do nothing without the consent of the parliament (Storting), which would not be convoked until there was a suspension of hostilities on the part of Sweden.

Sweden refused Christian's conditions and a short military campaign ensued in which the Norwegian army was defeated by the forces of the Swedish crown prince Charles John. The brief war concluded with the Convention of Moss on 14 August 1814. By the terms of this treaty, King Christian Frederick transferred executive power to the Storting, then abdicated and returned to Denmark. The Storting in its turn adopted the constitutional amendments necessary to allow for a personal union with Sweden and on 4 November elected Charles XIII of Sweden as the new king of Norway.

The couple was childless and lived in comparative retirement as leaders of the literary and scientific society of Copenhagen until Christian ascended the throne of Denmark. Christian's suspected democratic principles made him persona ingratissima at all the reactionary European courts, the court of Denmark included. It was not until 1831 that King Frederick gave Christian a seat on the council of state.

On 13 December 1839 he ascended the Danish throne as Christian VIII. The Liberal party had high hopes of “the giver of constitutions,” but he disappointed his admirers by steadily rejecting every Liberal project. Administrative reform was the only reform he would promise. In his attitude to the growing national unrest in the twin duchies of Schleswig and Holstein he often seemed hesitated and half-hearted, which damaged his position there. It was not until 1846 that he clearly supported the idea of Schleswig being a Danish area.

Some historians and biographers believe, however, that king Christian would have given Denmark a free constitution had he lived long enough, and his last words are sometimes (rather tragically) recorded as "I didn't make it". ("Jeg nåede det ikke.")

Seeing that his only legitimate son, the future Frederick VII, was apparently unable to beget heirs, he commenced arrangements to secure the succession in Denmark. The result was the selection of the future Christian IX as hereditary prince, the choice made official by a new law enacted on 31 July 1853 after an international treaty made in London.